Particle shower

In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that continues until many thousands, millions, or even billions of low-energy particles are produced. These are then stopped in the matter and absorbed.

Types of showers
The start of an electromagnetic shower.

There are two basic types of showers. Electromagnetic showers are produced by a particle that interacts primarily or exclusively via the electromagnetic force, usually a photon or electron. Hadronic showers are produced by hadrons (i.e. nucleons and other particles made of quarks), and proceed mostly via the strong nuclear force.

Electromagnetic showers

An electromagnetic shower begins when a high-energy electron, positron or photon enters a material. At high energies (above a few MeV, below which photoelectric effect and Compton scattering are dominant), photons interact with matter primarily via pair production — that is, they convert into an electron-positron pair, interacting with an atomic nucleus or electron in order to conserve momentum. High-energy electrons and positrons primarily emit photons, a process called bremsstrahlung. These two processes continue in turn, until the remaining particles have lower energy. Electrons and photons then lose energy via scattering until they are absorbed by atoms.

The "shower depth" is approximately determined by the relation

X = X0 ln(E0/Ec)/ ln2,

where X0 is the radiation length of the matter, and Ec is the critical energy. It logarithmically depends on the initial energy. The characteristic radius of the shower is the Molière radius.

Hadronic showers

A hadronic shower is produced by a high-energy hadron such as a nucleon, pion, or atomic nucleus. Some such particles have electric charge, and so produce showers that are partially electromagnetic, but all also interact with nuclei via the strong force. Although the details are more complex for this force, such an interaction involves one hadron interacting with a nucleus and producing several lower-energy hadrons. This continues, as with the electromagnetic shower, until all particles are stopped or absorbed in the material.

Examples of showers

Cosmic rays hit earth's atmosphere on a regular basis, and they produce showers as they proceed through the atmosphere. It was from these air showers that the first muons and pions were detected experimentally, and they are used today by a number of experiments as a means of observing ultra-high-energy cosmic rays. Some experiments, like Fly's Eye, have observed the visible atmospheric fluorescence produced at the peak intensity of the shower; others, like Haverah Park experiment, have detected the remains of a shower by sampling the energy deposited over a large area on the ground.

In particle detectors built at high-energy particle accelerators, a device called a calorimeter records the energy of particles by causing them to produce a shower and then measuring the energy deposited as a result. Many large modern detectors have both an electromagnetic calorimeter and a hadronic calorimeter, with each designed specially to produce that particular kind of shower and measure the energy of the associated type of particle.